Butene-1 copolymer tie layer in multilayer film structures having a low seal temperature and improved hot tack

ABSTRACT

Multilayer film (BOPP) structure comprising at least:
     A) a skin (outer) layer substantially consisting of a crystalline low seal Initiation temperature propylene copolymer;   B) a tie layer substantially consisting of a butene-1 copolymer having flexural modulus (MEF) of 75 MPa or less;   C) a core layer substantially consisting of one or more polypropylene homopolymers designed for BOPP.

The present invention relates to butene-1 copolymers, useful in the preparation of heat-sealable films, as tie layer in multilayer film structures.

Copolymers of propylene with other olefins (mainly ethylene, butene-1 or both), or mixtures of such copolymers with other olefin polymers are known in the prior art as heat-sealable materials.

These copolymers are obtained by polymerizing propylene with minor amounts of other olefin comonomers in the presence of coordination catalysts.

The polymerized comonomer units are statistically distributed in the resulting copolymer and the melting point of said copolymers results to be lower than the melting point of crystalline propylene homopolymers. Also the seal initiation temperature (as later defined in detail) of the said copolymers results to be favorably low.

However, particularly demanding applications of films, like form and fill packaging, require not only a low seal initiation temperature (hereinafter called “S.I.T.”), but also a good “hot tack”. As explained in U.S. Pat. No. 4,725,505, hot tack is the bonding strength measurable while the polymer in the heat sealed portion of a film is still in the semi-molten/solidifying state. Said form and fill packaging is commonly applied in the food packaging, especially for the production of bags to be used for solid and/or liquid products. The bags are produced with packaging machines that simultaneously seal the bottom seam of the bag and fills it while it is in the vertical or horizontal position. Thus the sealing, while still in the semi-molten/solidifying state, must be able to withstand the weight of the product introduced in the bag and generally also the pressure of air used to assist in transport of the product. According to the said U.S. Pat. No. 4,725,505, the hot tack is improved by adding at least 40% by weight of a butene-1-propylene copolymer to a propylene-ethylene copolymer. The hot tack strength values so obtained, measured by carrying out the test under air pressure, are in the range of 10-15 inch of water.

According to US2005/0142367, relatively high values of hot tack strength are achieved by blending a propylene-butene-1-ethylene terpolymer with a metallocene catalyzed ethylene polymer. The terpolymer used in the examples contains relatively high amounts of comonomers, namely 1.7 mol % of ethylene and 16.2 mol % of butene-1. The hot tack strength values obtained are lower than 250 g (about 2.5 N). At 210° F. (about 99° C.) it is lower than 200. It appears to be insufficient at temperatures lower than 200° F. (about 93° C.).

WO 2011/064124 and WO2011/064119 disclose polyolefin compositions particularly useful for the preparation of films, particularly multilayer films wherein at least one layer comprises a polyolefin compositions providing improved seal initiation temperature (S.I.T.) and hot tack properties. The layer comprising said composition acting as sealing layer (outermost layer) in the S.I.T. and hottack test.

In WO 2007/047133 multilayer film structures are disclosed including tie layers having thickness of 25 micron or less, wherein a first polymer present in a “core layer” of the invention is also present in the “tie layer” optionally in blend with a further tie layer polymer. The first polymer is contributing to the improvement of the seal strength and reduced minimum seal temperature of the multilayer structure over the control examples. The said first polymer can be a propylene or an ethylene based plastomer or elastomer or a butene-1 polymer (homo or copolymer).

It is still felt the need to improve hot tack and seal strength in multilayer structures. It has now surprisingly been found that an improvement of such properties and particularly valuable balance of heat-sealability (sufficiently low S.I.T.), and hot tack and seal strength (force) is obtained in bioriented polypropylene (BOPP) multilayer structures wherein at least one internal layer (tie layer) is substantially made of a butene-1 copolymer elastomer or plastomer.

Therefore the present invention provides bioriented polypropylene (BOPP) multilayer structure comprising at least:

-   A) a skin (outer, seal) layer substantially consisting of a     crystalline propylene copolymer composition comprising (percent by     weight):     -   a) 20-60 wt % of a copolymer of propylene with ethylene,         containing 1 to 5 wt % of ethylene; and     -   b) 40-80 wt % of a copolymer of propylene with ethylene and a         C4-C8 alpha-olefin, the ethylene content being 1 to 5 wt % and         the C4-C8 alpha-olefin content being 6 to 15 wt %;     -   the total content of ethylene in the composition being 1 to 5 wt         % and the total content of C4-C8 alpha-olefin in the composition         being 2.4 to 12 wt %.

The C4-C8 alpha-olefin is preferably of butene-1,1-pentene, 1-hexene, 4-methyl-1-pentene and 1-octene. Particularly preferred is butene-1. The above comonomer content (ethylene or alpha olefin) is referred to units in the polymer chain derived from the polymerized comonomer.

-   B) a tie layer substantially consisting of a butene-1 polymer having     -   a content of butene-1 derived units of 80 wt % or more,         preferably of 84 wt % or more     -   a flexural modulus (MEF) of 75 MPa or less, preferably of 40 MPa         or less, more preferably of from 10 to 30 MPa.; -   C) a core layer substantially consisting of polypropylene     homopolymers having melt flow rate higher than 2, preferably higher     than 5 g/10 min, more preferably of from 5 to 10 g/10 min and a     content of fraction insoluble in Xilene at room temperature (about     25° C.) of equal to or higher than 95 wt %.

The tie layer B is coextruded in between the skin and the core layers.

The core layer polymer is a high flow polypropylene homopolymer designed for the production of biaxially oriented polypropylene films (BOPP) on tubular double bubble lines and cast film.

The skin layer polymer, that is said crystalline propylene copolymer composition can be produced by conventional processes polymerizing propylene and, optionally, an alpha-olefin mentioned above in the presence of a suitable catalyst, such as a stereospecific Ziegler-Natta catalysts or a metallocene catalyst. The skin layer polymer can be prepared according to the process described in EP 674 991. The seal layer polymer (A) is a low seal Initiation temperature polymer preferably having at least one of the following properties:

-   -   a melting point from about 126° C. to 147° C.;     -   seal initiation temperature (as defined below) from 90° C. to         114° C.; and     -   a fraction soluble in n-hexane at 50° C. of less than 5.5% by         weight, preferably of from 3.5 to 4.5% by weight.

“Seal initiation temperature”, or S.I.T., (also referred to as heat-seal temperature) is the minimum temperature needed to form a seal of one polypropylene film layer to one film layer prepared from the composition of the seal layer polymer, so that the seal does not fail, i.e. the film layers do not separate at the seal, when a 2 N load is applied to this multilayer film. The particulars will be given in the examples.

The tie layer polymer is a butene-1 polymer, that is a homopolymer or a copolymer of butene-1 with at least one other alpha-olefin, preferably having at least one of the following properties:

-   -   molecular weight (Mw) higher than 100.000 more preferably higher         than 200.000 even more preferably higher than 300.000.     -   density of 0.895 g/cm³ or less, more preferably of from 0.865 to         0.895 g/cm³.     -   MFR at 190° C., 2.16 kg, of 0.3-10 g/10 min., in particular         0.3-5 g/10 min.

Preferably, the tie layer butene-1 polymer is selected from the group consisting of:

-   -   (b1) a butene-1 homopolymer or copolymer of butene-1 with at         least another alpha-olefin, preferably with propylene as         comonomer, having         -   content of butene-1 units in the form of isotactic             pentads (mmmm) from 25 to 55%;         -   intrinsic viscosity [η] measured in tetraline at 135° C.             from 1 to 3 dL/g;         -   content of xylene insoluble fraction lower than 60 wt % at             0° C., preferably of from 3 to 60 wt %; and preferably         -   Tm(II) of equal to or higher than 100° C.;     -   (b2) a butene-1/ethylene copolymer or a         butene-1/ethylene/propylene terpolymer, having the following         properties:         -   distribution of molecular weights (Mw/Mn) measured by GPC             lower than 3;         -   no melting point (TmII) detectable at the DSC measured             according to the DSC method described herein below;             preferably         -   intrinsic viscosity (I.V.) measured in tetrahydronaphtalene             (THN) at 135° C.>1.2 dL/g; more preferably         -   isotactic pentads (mmmm)>90%,

Component (b1) preferably is a copolymer having an amount of comonomer content (propylene derived units) in (b1) of from 3 to 5 wt %; even more preferably of from 3.5 to 4.5 wt % of comonomer.

Component (b2) preferably has a measurable melting enthalpy after aging. Particularly, measured after 10 days of aging at room temperature, the melting enthalpy of (b2) can be of less than 25 J/g, preferably of from 4 to 20 J/g. The butene-1 copolymer (b2) just after it has been melted does not show a melting point associated to polybutene-1 crystallinity, however it is crystallizable, i.e. after about 10 days that it has been melted the polymer shows measurable melting point and a melting enthalpy measured by DSC. In other words the butene-1 polymer has no melting temperature attributable to polybutene crystallinity (TmII) DSC, measured after cancelling the thermal history of the sample, according to the DSC method described herein below in the experimental part. The amount of comonomer (ethylene derived units) in (b2) is preferably of from 0.2 to 15% by mol, even more preferably of from 5 to 13% by mole corresponding to of from 0.1 to 8 wt %; even more preferably of from 2.5 to 7 wt % of comonomer.

The polymerization process for butene-1 (co)polymers (b1) and (b2) can be carried out according to known techniques, for example slurry polymerization using as diluent a liquid inert hydrocarbon, or solution polymerization using for example the liquid butene-1 as a reaction medium. Moreover, it may also be possible to carry out the polymerization process in the gas-phase, operating in one or more fluidized or mechanically agitated bed reactors. The polymerization carried out in the liquid butene-1 as a reaction medium is highly preferred. As a general rule, the polymerization temperature is generally comprised between −100° C. and +200° C., preferably from 20 to 120° C., more preferably from 40 to 90° C., most preferably from 50° C. to 80° C.

The polymerization pressure is generally comprised between 0.5 and 100 bar.

The polymerization can be carried out in one or more reactors that can work under same or different reaction conditions such as concentration of molecular weight regulator, comonomer concentration, temperature, pressure etc.

The butene-1 (co)polymer (b1) of the present invention can be prepared by polymerization of the monomers in the presence of a low stereospecificity Ziegler-Natta catalyst comprising (A) a solid component comprising a Ti compound and an internal electron-donor compound supported on MgCl₂; (B) an alkylaluminum compound and, optionally, (C) an external electron-donor compound. In a preferred aspect of the process for the preparation of the (co)polymers of the invention the external electron donor compound is not used in order not to increase the stereoregulating capability of the catalyst. In cases in which the external donor is used, its amount and modalities of use should be such as not to generate a too high amount of highly stereoregular polymer such as it is described in the International application WO2006/042815 A1. The butene-1 copolymers thus obtained typically have a content of butene-1 units in the form of isotactic pentads (mmmm) from 25 to 56%.

The butene-1 copolymer (b2) can be obtained polymerizing the monomer(s) in the presence of a metallocene catalyst system obtainable by contacting:

a stereorigid metallocene compound;

an alumoxane or a compound capable of forming an alkyl metallocene cation; and, optionally,

an organo aluminum compound.

Examples of the said catalyst system and of polymerization processes employing such catalyst system to obtain the butene-1 copolymer b2 can be found in WO2004/099269. Alternatively also suitable as butene-1 copolymer (b2) are those having higher comonomer content obtainable according to processes and conditions as described in and WO2009/000637.

The butene-1 copolymers as above said are also endowed with a high molecular weight, expressed in terms of intrinsic viscosity (IV) it is higher than 1 dl/g; preferably higher than 1.5. The intrinsic viscosity (IV) is preferably not higher than 3. Higher IV is associated with poor processability of the copolymer.

The butene-1 copolymers suitable for the use according to the invention have a relatively low crystallinity of less than 35% measured via X-ray, preferably of from 20 to 30%.

Preferably, the tie layer butene-1 polymer (b) has a glass transition temperature (Tg) measured via DMTA equal to or lower than −5° C., more preferably of from −10° C. to −5° C.

The tie layer polymer can optionally comprise, in addition to the butene-1 copolymer plastomer a small amount, preferably from 3-10%, more preferably from 6 to 8% by weight, of a crystalline propylene polymer added (e.g. by in-line compounding) to improve processability, reducing stickiness of the plastomer, without altering significantly the properties profile of the plastomer (e.g. ADSYL 5 C 30F, MIL 5.50 g/10 min @ 230° C./2.16 kg sold by Lyondell Basell).

Conventional additives, commonly used in olefin polymers, may be added, such as nucleating agents, anti-sticking agents, processing aids, stabilizing agents (against heat, light, U.V.) and other additives typical for film application such as plasticizers, antiacids, antistatic and water repellant agents, slip agents, antiblocking agents.

The particulars are given in the following examples, and detailed description which are given to illustrate, without limiting, the present invention.

Films according to the invention are at least comprising the A/B/C three layer structure as above detailed. The three-layer (A/B/C) film having a thickness of from 100 to 500 μm, typically about 150 μm (e.g. with a thickness ratio of the layers 50/50/50, different thickness ratio of the layers might be possible without departing from the scope of the present invention). Films according to the invention can be obtained by coextrusion on a three extruders extrusion line and then the coextruded films can be compression molded together with a homopolymer film having a thickness of about 1000 μm. The said homopolymer film having a thickness of about 1000 μm is made of a polypropylene homopolymer designed for the production of biaxially oriented polypropylene, that can be same or different from the core layer polymer C. After compression molding a thick 3 layers composite sheet is created (approx. 1100 μm thick). Thus the core layer in the composite film after compression molding is substantially summing up the layer C, as above defined, and the additional thicker homopolymer film (support). It is equivalent to obtain such composite sheets directly by coextrusion, with an extruder line of suitable capacity, extruding a much thicker core+support layer (e.g with a thickness ratio of the layers 50/50/1000). Thus, avoiding the compression moulding step.

Thus, a further object of the present invention is a three-layer (A/B/C) composite film, wherein layer C is made up of a core layer as above defined and/or a further thicker compression molded or coextruded support layer. Said support layer can be made of the same or a different core layer homopolymer designed for BOPP, such as above defined.

The final multilayer composite film is then cut to size and oriented via batch wise process. Biorientation can be obtained via industrial continuous processes known in the art such as tubular film process technologies and flat film (cast) process technologies. The final multilayer structures are generally characterized by a total thickness of less than 100 μm. Preferably from 20 to 50 μm.

Generally speaking, the films of this invention can be prepared by known techniques, such as extrusion and calendaring followed by orientation. Specific examples of films of the present invention are disclosed hereinafter in the test for determining the seal initiation temperature (S.I.T.) and the hot tack.

The following analytical methods have been used to determine the properties reported in the detailed description and in the examples.

Comonomer contents: determined by IR spectroscopy or by NMR (when specified). For the propylene copolymers the content of comonomer was determined via FT-IR after due calibration, analytical wavelengths:

-   -   C2 732 cm⁻¹     -   C4 765 cm⁻¹

For the tie layer butene-1 copolymers component (b) the amount of comonomers was calculated from ¹³C-NMR spectra of the copolymers of the examples. Measurements were performed on a polymer solution (8-12% by weight) in dideuterated 1,1,2,2-tetrachloro-ethane at 120° C. The ¹³C NMR spectra were acquired on a Bruker AV-600 spectrometer operating at 150.91 MHz in the Fourier transform mode at 120° C. using a 90° pulse, 15 seconds of delay between pulses and CPD (WALTZ16) to remove ¹H-¹³C coupling. About 1500 transients were stored in 32K data points using a spectral window of 60 ppm (0-60 ppm).

Copolymer Composition

Diad distribution is calculated from ¹³C NMR spectra using the following relations:

PP=100I ₁/Σ

PB=100I ₂/Σ

BB=100(I ₃ −I ₁₉)/Σ

PE=100(I ₅ +I ₆)/Σ

BE=100(I ₉ +I ₁₀)/Σ

EE=100(0.5(I ₁₅ +I ₆ +I ₁₀)+0.25(I ₁₄))/Σ

Where Σ=I₁+I₂+I₃−I₁₉+I₅+I₆+I₉+I₁₀+0.5(I₁₅+I₆+I₁₀)+0.25 (I₁₄)

The molar content is obtained from diads using the following relations:

P(m %)=PP+0.5(PE+PB)

B(m %)=BB+0.5(BE+PB)

E(m %)=EE+0.5(PE+BE)

I₁, I₂, I₃, I₅, I₆, I₉, I₁₀, I₁₄, I₁₅, I₁₉ are integrals of the peaks in the ¹³C NMR spectrum (peak of EEE sequence at 29.9 ppm as reference). The assignments of these peaks are made according to J. C. Randal, Macromol. Chem Phys., C29, 201 (1989), M. Kakugo, Y. Naito, K. Mizunuma and T. Miyatake, Macromolecules, 15, 1150, (1982), and H. N. Cheng, Journal of Polymer Science, Polymer Physics Edition, 21, 57 (1983). They are collected in Table A (nomenclature according to C. J. Carman, R. A. Harrington and C. E. Wilkes, Macromolecules, 10, 536 (1977)).

TABLE A I Chemical Shift (ppm) Carbon Sequence 1 47.34-45.60 S_(αα) PP 2 44.07-42.15 S_(αα) PB 3 40.10-39.12 S_(αα) BB 4 39.59 T_(δδ) EBE 5 38.66-37.66 S_(αγ) PEP 6 37.66-37.32 S_(αδ) PEE 7 37.24 T_(βδ) BBE 8 35.22-34.85 T_(ββ) XBX 9 34.85-34.49 S_(αγ) BBE 10 34.49-34.00 S_(αδ) BEE 11 33.17 T_(δδ) EPE 12 30.91-30.82 T_(βδ) XPE 13 30.78-30.62 S_(γγ) XEEX 14 30.52-30.14 S_(γδ) XEEE 15 29.87 S_(δδ) EEE 16 28.76 T_(ββ) XPX 17 28.28-27.54 2B₂ XBX 18 27.54-26.81 S_(βδ) + 2B₂ BE, PE, BBE 19 26.67 2B₂ EBE 20 24.64-24.14 S_(ββ) XEX 21 21.80-19.50 CH₃ P 22 11.01-10.79 CH₃ B

N-hexane extractables: Determined by suspending in an excess of hexane a 100 micrometer thick film specimen of the composition being analyzed, in an excess of hexane, in an autoclave at 50° C. for 2 hours. Then the hexane is evaporated and the dried residue is weighted.

Melt Flow Rate MFR

Determined according to ISO 1133, at 230° C., 2.16 kg load for propylene polymers, at 190° C., 2.16 kg load for butene-1 and ethylene polymers.

Intrinsic Viscosity [η]: Measured in tetrahydronaphthalene (THN, tetralin) at 135° C.

Flexural modulus: Determined according to ISO method 178 on compression molded plaques prepared according to ISO8986.

Tensile properties (Tensile Stress at Break, Elongation at Break, Stress at Yield, Elongation at Yield): Determined according to ISO 527-1,-2 on compression molded plaques prepared according to ISO8986.

Tension set Determined according to ISO 2285 on compression molded plaques prepared according to ISO8986.

Hardness (Shore A) Determined according to ISO 868 on compression molded plaques prepared according to ISO8986.

Tg Determination Via DMTA Analysis

Compression Molded specimen of 76 mm by 13 mm by 1 mm are fixed to the DMTA machine for tensile stress. The frequency of the tension and relies of the sample is fixed at 1 Hz. The DMTA translate the elastic response of the specimen starting form −100° C. to 130° C. In this way it is possible to plot the elastic response versus temperature. The elastic modulus for a viscoelastic material is defined as E=E′+iE″. The DMTA can split the two components E′ and E″ by their resonance and plot E′ vs temperature and E′/E″=tan (δ) vs temperature.

The glass transition temperature Tg is assumed to be the temperature at the maximum of the curve E′/E″=tan (δ) vs temperature.

Determination of X-Ray Crystallinity

The X-ray crystallinity was measured with an X-ray Diffraction Powder Diffractometer using the Cu-Kα1 radiation with fixed slits and collecting spectra between diffraction angle 2Θ=5° and 2Θ=35° with step of 0.1° every 6 seconds.

Measurement were performed on compression molded specimens in the form of disks of about 1.5-2.5 min of thickness and 2.5-4.0 cm of diameter. These specimens are obtained in a compression molding press at a temperature of 200° C.±5° C. without any appreciable applied pressure for 10 minutes. Then applying a pressure of about 10 Kg/cm² for about few second and repeating this last operation for 3 times.

The diffraction pattern was used to derive all the components necessary for the degree of crystallinity by defining a suitable linear baseline for the whole spectrum and calculating the total area (Ta), expressed in counts/sec·2Θ, between the spectrum profile and the baseline. Then a suitable amorphous profile was defined, along the whole spectrum, that separate, according to the two phase model, the amorphous regions from the crystalline ones. Thus it is possible to calculate the amorphous area (Aa), expressed in counts/sec·2Θ, as the area between the amorphous profile and the baseline; and the crystalline area (Ca), expressed in counts/sec·2Θ, as Ca=Ta−Aa

The degree of crystallinity of the sample was then calculated according to the formula:

% Cr=100×Ca/Ta

The Melting Temperature (Tm) of Propylene Polymers—(ISO 11357-3)

Determined by differential scanning calorimetry (DSC). A sample weighting 6±1 mg, is heated to 200±1° C. at a rate of 20° C./min and kept at 200±1° C. for 2 minutes in nitrogen stream and it is thereafter cooled at a rate of 20° C./min to 40±2° C., thereby kept at this temperature for 2 min to crystallise the sample. Then, the sample is again heated at a temperature rise rate of 20° C./min up to 200° C.±1. The heating scans are recorded, thermograms are obtained, and, from this, temperatures corresponding to peaks are read. The melting point can be determined either in the first or in the second heating run, or in both the two runs. It is preferably determined in the second run for propylene polymers. The temperature corresponding to the most intense melting peak recorded during the relevant heating run is taken as the melting temperature.

The melting temperatures of the tie layer (b) butene-1 polymers (TmII) were measured by Differential Scanning calorimetry (D.S.C.) on an Perkin Elmer DSC-7 instrument, according to the following method.

A weighted sample (5-10 mg) obtained from the polymerization was sealed into aluminum pans and heated at 200° C. with a scanning speed corresponding to 20° C./minute. The sample was kept at 200° C. for 5 minutes to allow a complete melting of all the crystallites. Successively, after cooling to −20° C. with a scanning speed corresponding to 10° C./minute, the peak temperature was taken as crystallization temperature (Tc). After standing 5 minutes at −20° C., the sample was heated for the second time at 200° C. with a scanning speed corresponding to 10° C./min. In this second heating run, the peak temperature was taken as the melting temperature (TmII) of crystalline form 2 (the first to form, being favoured kinetically) and the area as global melting enthalpy (ΔHfII).

The melting enthalpy after 10 days was measured as follows by using the Differential Scanning calorimetry (D.S.C.) on an Perkin Elmer DSC-7 instrument. A weighted sample (5-10 mg) obtained from the polymerization was sealed into aluminum pans and heated at 200° C. with a scanning speed corresponding to 20° C./minute. The sample was kept at 200° C. for 5 minutes to allow a complete melting of all the crystallites. The sample was then stored for 10 days at room temperature. After 10 days the sample was subjected to DSC, it was cooled to −20° C., and then it was heated at 200° C. with a scanning speed corresponding to 10° C./min. In this heating run, the peak temperature was taken as the melting temperature (Tm) and the area as global melting enthalpy after 10 days (ΔHf).

Molecular Weight and Molecular Weight Distribution (MWD):

Measured by way of gel permeation chromatography (GPC) in 1,2,4-trichlorobenzene. Molecular weight parameters ( M _(n), M _(w), M _(z)) and molecular weight distributions (e.g. MWD=Mw/Mn) for all the samples were measured using a Waters GPCV 2000 apparatus, which was equipped with a column set of four PLgel Olexis mixed-gel (Polymer Laboratories) and an IR4 infrared detector (PolymerChar). The dimensions of the columns were 300×7.5 mm and their particle size 13 μm. The mobile phase used was 1-2-4-trichlorobenzene (TCB) and its flow rate was kept at 1.0 mL/min. All the measurements were carried out at 150° C. Solution concentrations were 0.1 g/dL in TCB and 0.1 g/L of 2,6-diterbuthyl-p-chresole were added to prevent degradation. For GPC calculation, a universal calibration curve was obtained using 10 polystyrene (PS) standard samples supplied by Polymer Laboratories (peak molecular weights ranging from 580 to 8500000). A third order polynomial fit was used for interpolate the experimental data and obtain the relevant calibration curve. Data acquisition and processing was done using Empower (Waters). The Mark-Houwink relationship was used to determine the molecular weight distribution and the relevant average molecular weights: the K values were K_(PS)=1.21×10⁻⁴ dL/g and K_(PB)=1.78×10⁻⁴ dL/g for PS and PB respectively, while the Mark-Houwink exponents α=0.706 for PS and α=0.725 for PB were used.

For butene/ethylene copolymers, as far as the data evaluation is concerned, it was assumed for each sample that the composition was constant in the whole range of molecular weight and the K value of the Mark-Houwink relationship was calculated using a linear combination as reported below:

K _(EB) =x _(E) K _(PE) +x _(P) K _(PB)

where K_(EB) is the constant of the copolymer, K_(PE) (4.06×10⁻⁴, dL/g) and K_(PB) (1.78×10⁻⁴ dL/g) are the constants of polyethylene and polybutene, x_(E) and x_(B) are the ethylene and the butene wt % content. The Mark-Houwink exponents α=0.725 was used for all the butene/ethylene copolymers independently on their composition.

For butene/propylene copolymers, as PP and PB have very similar K, no corrections were applied and the copolymer was integrated using the K and a values of PB.

Density: According to ISO 1183. The method ISO is based on observing the level to which a test specimen sinks in a liquid column exhibiting a density gradient.

Standard specimen are cut from strands extruded from a grader (MFR measurement). The polybutene-1 specimen is putted in an autoclave at 2000 bar for 10 min at a room temperature in order to accelerate the transformation phase of the polybutene. After this the specimen is inserted in the gradient column where density is measured according to ISO 1183.

Xylene Soluble and Insoluble Fraction—Propylene Polymers

Determined as follows.

2.5 g of polymer and 250 cm³ of xylene are introduced in a glass flask equipped with a refrigerator and a magnetical stirrer. The temperature is raised in 30 minutes up to the boiling point of the solvent. The so obtained clear solution is then kept under reflux and stirring for further 30 minutes. The closed flask is then kept for 30 minutes in a bath of ice and water and in thermostatic water bath at 25° C. for 30 minutes as well. The so formed solid is filtered on quick filtering paper. 100 cm³ of the filtered liquid is poured in a previously weighed aluminum container which is heated on a heating plate under nitrogen flow, to remove the solvent by evaporation. The container is then kept in an oven at 80° C. under vacuum until constant weight is obtained.

Xylene Soluble and Insoluble Fraction at 0° C.—Butene-1 Polymers

Determined as follows.

2.5 g of polymer are dissolved in 250 ml of xylene, at 135° C., under agitation. After 20 minutes, the solution is cooled to 0° C. under stirring, and then it is allowed to settle for 30 minutes. The precipitate is filtered with filter paper; the solution is evaporated under a nitrogen current, and the residue dried under vacuum at 140° C. until constant weight. The weight percentage of polymer soluble in xylene at 0° C. is then calculated.

¹³C-NMR Isotacticity Index—Butene-1 Polymers

40 mg of each sample are dissolved in 0.5 mL of C₂D₂Cl₄.

The ¹³C NMR spectra are acquired on a Bruker AV-600 equipped with cryoprobe (150.91 Mhz, 90° pulse, 15 s delay between pulses). About 512 transients are stored for each spectrum; mmmm pentad peak (27.73 ppm) is used as reference.

The microstructure analysis is carried out as described in literature (Macromolecules 1991, 24, 2334-2340, by Asakura T. et Al. and Polymer, 1994, 35, 339, by Chujo R. et Al.). The percentage value of pentad tacticity (mmmm %) is the percentage of stereoregular pentads (isotactic pentad) as calculated from the relevant pentad signals (peak areas) in the NMR region of branched methylene carbons (around 27.73 ppm assigned to the BBBBB isotactic sequence), with due consideration of the superposition between stereoirregular pentads and of those signals, falling in the same region, due to the alfa-olefin comonomer.

Determination of the Hot Tack Strength and of the Seal Strength and SIT

For the hot tack strength measurements, the following method is followed (ASTM F 1921).

For each test film specimens 15 mm wide are superimposed in alignment, the adjacent layers being layers of the particular test composition.

The seals are made at increasing temperatures (5° C. steps) with a J&B Hot tack tester 3000 sealer, equipped with brass teflon coated sealing bars, at one end of the said superimposed specimens along the 15 mm side and the hot tack strength is measured by peeling the specimens at a speed of 100 mm/sec.

The sealing conditions are:

-   -   sealing pressure of 0.1 MPa (14.5 psi);     -   dwell time of 0.5 sec

The test is carried out immediately after sealing (0.2 sec). The hot tack strength is given by the load required to separate the sealed specimens.

For the seal strength measurements, the superimposed specimens are sealed along one of the 15 mm sides with a RDM HSE-3 five bars sealer type. Sealing time is 0.5 seconds at a pressure of 0.1 MPa (14.5 psi). The sealing temperature is increased for each seal with steps of 5° C., starting from a sufficiently low temperature to make it possible to determine a significant level of sealing force. The sealed samples are left to cool for 24 hours and then their unsealed ends are attached to an Instron machine (4301 model) where they are tested at a crosshead speed of 100 mm/min (grip distance 50 mm) Reference is standard ASTM F 88.

The S.I.T. (seal initiation temperature) is the minimum sealing temperature at which the seal shows a sealing force of 2.0 Newton in the above said test conditions. The sealing temperature is increased for each seal with steps of 2° C.

EXAMPLES 1 AND 2 AND COMPARISON EXAMPLE 3C 4C AND REFERENCE EXAMPLE

The following materials are used as polymer components of layer A), B) and C) and for the additional support layer (homopolymer film as above said coextruded or compression molded or calendered on to the free surface of core layer C)

A) Seal Layer Polymer

PP-1 low S.I.T. crystalline propylene terpolymer blend having MFR 5.50 g/10 min at 230° C./2.16 kg, melting temperature DSC Tm of 132.3° C.; Seal initiation temperature 105° C., total content of ethylene derived units 3.2 wt %, and total content of butene-1 derived units abt. 6 wt %., prepared according to the process and procedure in example 1 of WO0674991.

PP-1 is made of

35 wt % of a propylene-ethylene copolymer having 3.2 wt % of ethylene derived units; and

65 wt % of a propylene ethylene butene-1 terpolymer having 3.3 wt % of ethylene derived units and 9.2 wt % of butene-1 derived units.

B) Tie Layer Polymer

PB1 is a butene-1 copolymer with propylene produced with Ziegler Natta catalyst in absence of external donor according to the process described in the International application WO2006/042815 A1.

PB2 is a butene-1/ethylene copolymer produced according to the process described in WO2004/099269.

The butene-1 copolymers of the examples were pelletized and dried with use of commercial additives, antisticking agents to improve flowability and contributing to the processability of the compositions. Finishing treatments lead to a total amount of additives in the final pellets typically less than 10 wt %, preferably less than 1.0 wt %, preferably less than 0.5 wt % even more preferably less than 0.2 wt % (about 100-1500 ppm per additive or less). Examples of such finishing treatments can be found in the international patent application PCT/EP2010/056159.

Table 1 is reporting the structures and properties of butene-1 copolymers used in the examples for the tie layer (b) according to the invention

TABLE 1 PB1 PB2 Plastomer type b1 b2 C4C3 C4C2 C3 content (NMR) wt % 3.9 — C2 content (NMR) wt % — 4.8 (IR 5.4)** Intrinsic Viscosity dl/g 2.3 1.95 Melt Flow Rate - @ g/10 min 0.45 1 190/2.16 Density g/cc 0.8786 0.8830 Flexural elastic modulus MPa 31 75 (ISO 178) Hardness Shore A 74.5 89.4 (ISO 868) Tg (DMTA) ° C. −5.8 −22 % cristall. RX % 29 25 DSC Tm II* ° C. 100 nd DSC Tm I 118 49 S.X.0/0° C. Soluble Total wt % 57 99 mmmm % % 54 90 Mw/Mn 6.1 2.8 ΔHf after 10 days J/g — 26.97 1, 4 insertions absent na Strain at break ISO527 410 550 Stress at break ISO527 12.9 16.9 Tension set, 100% at 23° C. 39 58 Nd = not detectable Na = data not available *from DSC thermogram collected in second heating run (after cancelling the thermal history of the sample) **IR analytical wavelength abt. 727 cm−1

C) Core Layer Polymer

PP-2 is a polypropylene homopolymers having Melt flow rate (MFR) 8.0 g/10 min (230° C./2.16 Kg), fraction Soluble in Xilene (SX) at room temperature (25° C.) of 3.8 wt %.

PP-3 used as further homopolymer support layer (coextruded or compression molded on the external surface of layer C). PP-3 is a polypropylene homopolymer having Melt flow rate (MFR) of 2 g/10 min (230° C./2.16 Kg), a content of fraction Soluble in Xilene (SX) at room temperature (25° C.) of 4 wt %.

Film Preparation

BOPP films were produced using a multi-step process.

Firstly a three-layer (A/B/C) film having a thickness of about 150 μm (50/50/50) is coextruded on a three extruders Dr. Collin extrusion line; then the coextruded films were compression molded together with a homopolymer film having a thickness of about 1000 μm, The propylene homopolymer film was made of PP-3., to create a thick 3 layers composite sheet (approx. 1100 μm) (PP-2 and PP-3 layers being substantially counted as one uniform core/support layer after compression molding). These composite sheets were then cut to size and oriented via batch wise process using a Brukner KARO IV Film Stretcher. The Brukner KARO IV Film Stretcher is a lab scale stretching device. Similar orientation conditions are used for each material. The extrusion, compression molding and stretching conditions are provided below.

Extrusion Conditions:

-   -   Dr. Collin Extruder line; 30 mm, 30 L/D (layer A):         Cylinder #1 (set) 180° C.;         Cylinder #2 (set) 190° C.;         Cylinder #3 (set) 200° C.;         Cylinder #4 (set) 210° C.;         Adapter #1 (set) 220° C.;         Screw speed 90-95 rpm         Melt temperature 210-220° C.     -   Dr. Collin Extruder line; 30 mm, 30 L/D (layer C):         Cylinder #1 (set) 180° C.;         Cylinder #2 (set) 190° C.;         Cylinder #3 (set) 200° C.;         Cylinder #4 (set) 210° C.;         Adapter #1 (set) 220° C.;         Screw speed 90-95 rpm         Melt temperature 210-220° C.     -   Dr. Collin Extruder line; 45 mm, 30 L/D (layer B):         Cylinder #1 (set) 190° C.;         Cylinder #2 (set) 210° C.;         Cylinder #3 (set) 230° C.;         Cylinder #4 (set) 230° C.;         Adapter #1 (set) 230° C.;         Screw speed 20-26 rpm         Melt temperature 175-236° C.         Cast roll unit speed 7-8 m/min;         Cast roll surface temperature 20° C.;

Compression Molding Conditions:

PHI Compression Molder was Used:

-   -   Superimposed films of said homopolymers PP-3 and heat seal         material three layer A/B/C film, were placed between sheets of         Mylar film then between two steel platens;     -   Platen sandwich placed in the PHI press at 200° C. for 2 minutes         at 3 tons of pressure;     -   Platen sandwich removed from press and inserted into another         press set at 22-25° C. for 2 additional minutes at 3 tons         pressure;     -   Platens separated and sample removed.

Stretching Conditions KARO IV Film Stretcher was Used:

-   -   Sample cut from compression molded sheet;     -   Sample loaded into KARO IV Film Stretcher at 160° C., allowed a         30 second preheat, and then stretched 6× by 6× at a rate of         84%/sec;     -   Sample removed from unit and allowed to cool.

The final composite multilayer films having an A/B/C structure, wherein in the films of Examples 1 and 2 layer B was made of PB1 or PB2 respectively, while layer A was made of PP-1, and layer C was made of PP-2+ thick support layer made of PP-3. The final thickness of the bioriented film (after stretching) was abt. 30 μm and the thickness ratio of the layers is about 4/4/92.

In the comparative examples only the tie layer polymer is changed.

Film A (Reference example), the same PP-2 homopolymer is used as tie layer polymer.

Film D (comparative example 3C), an heterophasic polypropylene soft composition HECO1 is used as tie layer. HECO1 is a polymer composition (reactor blend) having MFR of 5.2 g/10 min (230° C./2.16 Kg), made of

-   -   46 wt % of a random copolymer of propylene matrix having MFR 65         g/10 min (230° C./2.16 Kg), Tm (DSC) 138° C., amount of ethylene         derived units of 3.5 wt %; and     -   54 wt % of a propylene/ethylene copolymer rubber (bipo) having         Intrinsic Viscosity of 2.1 dl/g.

Film E (comparative example 4C), a polymer PP-4 is used as tie layer. PP-4 is a Random propylene/butene-1 copolymer composition having Melt flow rate (MFR) 5.5 g/10 min (230° C./2.16 Kg), Tm Melting temperature 137° C. and a SIT Seal initiation temperature 112° C.; made of

-   -   70 wt % of a propylene butene-1 copolymer, amount of butene-1         derived units 9.5 wt %     -   30 wt % of a propylene butene-1 copolymer, amount of butene-1         derived units 16.0 wt %

The properties of the films so obtained are reported in Table 2.

From Table 2 it is evident that the tie layer polymer of the present invention provide a satisfactory reduction of the SIT value with respect to the reference (simulating absence of the tie layer) and comparative 3C and 4C (different low SIT soft material in the tie layer). The seal strength is stable and high (above 4 N) in a temperature range from at least 110 to 130° C. The value of hot tack strength is also satisfactory in the range between 115 to 140° C., and such value undergoes only minor variations (stable in sealing window) with both PB1 and PB2.

Particularly multilayer structures comprising PB1 as tie layer (plastomer obtained with ZN catalyst) exhibit excellent behavior, seal strength stability (above 5N over 110° C.) and higher values of hot tack strength (well above 2 N) in the range between 110 to 140° C.

TABLE 2 Thickness Micron Film A Example Film D Film E (in coextrusion Reference (no tie Film B Film C Example 3C Example 4C Structure step) layer polymer) Example 1 Example 2 (comparative) (comparative) First skin 50 PP-1 PP-1 PP-1 PP-1 PP-1 layer C3C4C2 C3C4C2 C3C4C2 C3C4C2 C3C4C2 Tie layer 50 PP-2 PB1 PB2 HECO1 PP-4 C3 C4C3 C4C2 C3C2 C3C4C2 Core 50 PP-2 PP-2 PP-2 PP-2 PP-2 Layer C3 C3 C3 C3 C3 MaxForce (N) MaxForce (N) MaxForce (N) MaxForce (N) MaxForce (N) Temp. ° C. A B C D E Hot Tack 95 0.00 0.00 0.00 0.00 0.00 Strengh 100 0.30 1.00 1.06 0.30 0.57 105 1.11 1.78 1.83 1.13 1.55 110 2.24 2.13 1.92 1.55 2.42 115 2.67 2.57 1.97 2.24 2.85 120 2.50 3.02 1.93 2.37 3.08 125 2.11 2.99 1.72 2.13 2.75 130 2.01 2.45 1.80 1.46 2.80 135 2.15 2.16 2.26 1.29 2.47 140 1.76 2.12 0.79 1.30 2.42 145 1.68 150 1.73 1.83 1.51 2.03 160 1.55 2.64 1.49 2.29 170 1.61 1.87 0.44 1.24 180 1.13 1.27 Sealing 95 0.2 0.1 Curve 100 0.4 0.2 0.9 0.2 0.1 105 3.6 1.9 3.8 1.5 1.7 110 3.4 5.2 5.6 3.6 2.8 120 3.0 5.5 4.0 4.0 3.5 130 2.9 5.7 5.1 4.2 4.1 5.3 4.9 4.2 4.2 SIT (° C.) 103 106 106 108 109 Brugger 

What is claimed is:
 1. multilayer structure comprising at least: A) a skin layer comprising a crystalline propylene copolymer composition comprising (percent by weight): a) 20-60 wt % of a copolymer of propylene with ethylene. containing 1 to 5 wt % of ethylene; and b) 40-80 wt % of a copolymer of propylene with ethylene and a C4-C8 alpha-olefin, wherein the ethylene content is 1 to 5 wt % and the C4-C8 alpha-olefin content is 6 to 15 wt %; wherein the total content of ethylene in the composition A is in a range of 1 to 5 wt % and the total content of C4-C8 alpha-olefin in the composition is in a range of 2.4 to 12 wt %. B) a tie layer comprising a butene-1 polymer having: a content of butene-1 derived units of 80 wt % or more, and a flexural modulus of equal to 75 MPa or less; C) a core layer comprising one or more polypropylene homopolymers having a melt flow rate of greater than 2 g/10 min and a content of xylene insoluble fraction at room temperature of equal to or greater than 95 wt %.
 2. The bioriented multilayer structure of claim 1 wherein the tie layer butene-1 polymer is selected from the group consisting of: (b1) a butene-1 homopolymer or copolymer of butene-1 with at least another alpha-olefin with propylene as comonomer comprising: a content of butene-1 units in the form of isotactic pentads (mmmm) from 25 to 55%; an intrinsic viscosity [η] measured in tetraline at 135° C. from 1 to 3 dL/g; a content of xylene insoluble fraction equal to or less than 60 wt % at 0° C.; and (b2) a butene-1/ethylene copolymer or a butene-1/ethylene/propylene terpolymer comprising: a Mw/Mn measured by GPC of equal to or less than 3; no melting point (TmII) that is detectable at the DSC.
 3. The bioriented multilayer structure of claim 1, wherein layer C comprises a core layer and/or a further thicker compression molded or a coextruded support layer made of the same or a different core layer homopolymer. 